1. Field of the Invention
The invention relates to tensile loads on flexible links, such as cables and chains, and the transfer of at least a portion of the load to an element other than the flexible link while the load is supported. More specifically, the invention relates to systems supporting a load with a flexible link, such as a floatation system with a buoyancy can supporting a riser or a system for mooring an offshore platform to a piling, and transferring at least a portion of the load from the flexible link to another element while the system continues to support the load.
2. Description of the Related Art
FIG. 1 is a schematic side view of a prior art floatation system having a buoyancy can supporting a subsea riser. A known floatation system 2 described in PCT Publication No. WO 2009/156695 A1 supports a subsea production riser 4 coupled to an underground well 6 below the seabed 8. The riser 4 is connected to a catenary production line 10 that provides a conduit for oil and gas fluids to a storage tank 12 floating on the surface 14 of the sea. The floatation system 2 includes a buoyancy can 16 with a chain, cable or other flexible link 18 connected to a riser support connection 20. The riser connection 20 in turn is connected to the riser 4.
The stability and operational integrity of the riser 4 depends on the buoyancy can 16 and the flexible link 18 connected between the can and the riser. Periodically, the flexible link 18 may need replacement or repair. However, with the buoyancy can 16 placed in service with a tensile load on the flexible link, replacement or repair of the flexible link can be difficult. The flexible link cannot be removed without compromising the function of the buoyancy can to support the riser.
FIG. 2A is a schematic perspective view of an alternative prior art floatation system with multiple cables. FIG. 2B is a schematic detail perspective view of an upper connector. FIG. 2C is a schematic detail perspective view of a lower connector. The figures will be described in conjunction with each other. One alternative system disclosed in PCT Publication No. WO 2011/007084 A1 provides multiple cables 42A, 42B, 42C (generally “42”) connected between the buoyancy can 16 and the riser support connection 20. One end of the cables 42 is connected to an upper connector 22 that is connected to the buoyancy can 16. The other end of the cables is connected to a lower connector 36 that is connected to the riser support connection 20, which in turn is connected to the riser 4. The upper connector 22 has protruding lugs 24 that are formed with an end cap 26 and a shoulder 28 having a reduced cross sectional area compared to the end cap. The upper connector 22 also has a centrally formed upper keyhole 30 having an opening 32 and a slot 34 below the opening with a smaller cross sectional distance than the opening 32. Similarly, the lower connector 36 has protruding lugs 38 that are formed with an end cap and a shoulder, and a lower keyhole 40. The cables 42 are removably connected between upper and lower lugs 24, 38. The floatation of the buoyancy can 16 and the weight of the riser 4 provides tension to the cables, so that the cables 42 remain connected between the lugs 24, 38 in normal operation.
FIG. 3A is a schematic side view of the prior art floatation system of FIGS. 2A-2C with a broken cable. FIG. 3B is a schematic side view of the system of FIG. 3A with an ROV initiating repair functions. FIG. 3C is a schematic side view of a hydraulic cylinder and tether installed for repairs of the system of FIG. 3A. FIG. 3D is a schematic side view of an ROV supplying a replacement cable to the system of FIG. 3A. FIG. 3E is a schematic side view of the ROV removing the hydraulic cylinder and tether after repair. The figures will be described in conjunction with each other.
If a cable, such as cable 42C, needs repair or replacement, the remaining cables 42A, 42B maintain the floatation system 2 during repairs. A remote operated vehicle (“ROV”) 44 supplies a tether 46 and a hydraulic cylinder 48 to the floatation system. The ROV 44 attaches an upper tether end 50 of the tether 46 into the upper keyhole 30 of the upper connector 22 and the hydraulic cylinder to the lower keyhole 40 of the lower connector 36. The hydraulic cylinder is pressurized to tension the tether and pull the two connectors 22, 36 toward each other. The ROV supplies a new cable 42D and places the new cable on the lugs 24, 38 of the connectors 22, 36, respectively, while the cylinder 48 is actuated and the two connectors are pulled closer. The hydraulic cylinder pressure is released to deactivate the cylinder, and the two connectors 22, 36 are allowed to separate so that the cables 42 are tensioned again. The tether and hydraulic cylinder are removed, and the system can resume normal operational status.
FIG. 4 is a perspective view of another alternative prior art floatation system. The floatation system is similar to the one described in references to the above figures, but further includes two tethers 46 and two hydraulic cylinders 48, one set on each side of the cables 42. The tethers are each illustrated having two tether portions 46A, 46B. Separate upper lugs 52A, 52B laterally disposed on the buoyancy can 16 and separate lower lugs 54A, 54B laterally disposed on the riser support connection 20 are used for the tethers and hydraulic cylinders instead of the keyholes 30, 40 on the connectors 22, 36, respectively.
Despite the various features of the above described floatation systems, the systems rely on hydraulic pressure in a hydraulic cylinder to remain constant when activated during the repair operations. If the fluid leaks and the pressure decreases, then the connectors separate and the repair may not be completed. The hydraulic system not maintaining pressure under such adverse conditions can compromise the repair operations. The expense can be significant with the costs of the ROV rental, support ship, downtime, and related costs.
Other systems with a tensile load thereon have similar challenges. As another example, mooring lines between an offshore platform and a pile in the seabed can require repair. Yet, the logistics of transferring the load from the mooring line for a period of time during the repair can be challenging, as described more fully in WO 2010/127220. The location of the system, size, and accessibility can cause significant expense in replacing or repairing a member that supports the load between two bodies.
Therefore, there remains a need to provide an improved system and method for repair and other transfers from a tensioned cable.